The present invention relates to a thermally conductive polymer sheet that has excellent thermal conductivity. More particularly, the invention relates to a thermally conductive polymer sheet including specific graphitized carbon fibers for effectively diffusing heat from a variety of components of electronic hardware such as semiconductor elements, a power supply, a light source, and other parts.
With recent advancements, miniaturization, and lightening of electronic hardware, semiconductor packages have become more compact and more highly integrated and operated at higher speed. Therefore, heat generated by the electronic hardware is a very important issue. Generally, to remove the heat from heat-generating components, a sheet material made of a thermally conductive polymer composition is placed between the heat source and a radiator or between the heat source and a metal heat-transfer plate.
Such polymer compositions include a polymer matrix such as resin and rubber and a filler that has high thermal conductivity in the polymer matrix. Possible fillers include metal oxide, metal nitride, metal carbide, and metal hydroxide such as aluminum oxide, boron nitride, aluminum nitride, silicon nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and aluminum hydroxide. However, such compositions do not necessarily have sufficient thermal conductivity.
To improve thermal conductivity, various thermally conductive compositions that are filled with carbon fibers or graphite powders of high thermal conductivity have been proposed.
Japanese Unexamined Patent Publication No. 9-283955 discloses a thermally conductive sheet in which graphitized carbon fibers of particular aspect ratio are dispersed in a polymer such as silicone.
In Japanese Unexamined Patent Publication No. 2000-195998, we disclosed a polymer sheet that has good thermal conductivity, a method for producing the sheet, and a semiconductor device having the sheet that achieves a good heat transfer. The sheet is a silicon rubber sheet and it includes pitch-based carbon fibers that are coated with ferromagnetic material and oriented in a certain direction.
Advanced electronic parts such as a semiconductor element generate a large amount of heat. This may lead to various problems such as acceleration of electrochemical migration, development of corrosion at wiring or a pad, cracks or breakdown in a component due to thermal stress, reliability or durability loss of electronic parts due to detachment of jointed surfaces of components. Therefore, there is a need for a composition which has much higher thermal conductivity. The thermal conductivities of conventional thermally conductive compositions as described above are inadequate.
For example, Japanese Unexamined Patent Publication No. 9-283955 discloses a graphitized carbon fiber that serves as a thermally conductive filler. The graphitized carbon fiber is made by carbonizing, graphitizing, and pulverizing raw carbon fiber. After the pulverization, cracks occur in the axial direction of the fiber. A condensation polymerization reaction and a cyclization reaction during graphitization are slow to proceed. In addition, the ratio of the cross-sectional area of the cracked surfaces of the pulverized fibers to the total cross-sectional area is large. Therefore, thermal conductivity of the resulting thermally conductive sheet is inadequate.
In addition, in relation with miniaturization and lightening of electronic parts, a thermally conductive molded article which has an excellent thermal conductivity in a certain direction is desired. For example, a thermally conductive polymer sheet that is placed between electronic parts of a semiconductor device requires high thermal conductivity in the direction perpendicular to the sheet.
An object of the present invention is to provide a thermally conductive polymer sheet that can diffuse a large amount of heat from electric or electronic parts and that has good thermal conductivity in the direction perpendicular to the sheet.
A thermally conductive polymer sheet includes a polymer matrix and graphitized carbon fibers, which are a thermally conductive filler. The fibers are obtained by spinning, infusibilizing, carbonizing, pulverizing, and then graphitizing mesophase pitch. The fibers have a coating layer of a ferromagnetic material, The fibers are oriented to be perpendicular to the sheet.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.